Unpacking the plant-based matryoshka

Abstract

The environmental impact of animal agriculture has increased the demand for more sustainable alternatives to conventional meat products. Plant-based meat alternatives offer a promising option, but reproducing the characteristic fibrous structure and texture of meat remains challenging. High-moisture extrusion (HME) is widely used to create fibrous structures in plant-based meat alternatives, yet the mechanisms underlying structure formation across different length scales remain insufficiently understood. The aim of this thesis was to investigate how multiscale structure develops during HME of plant-based proteins, with particular emphasis on nm-to-µm scale structural rearrangements and their role in larger scale structure formation and the final texture of the extrudates. To achieve this aim, SAS techniques were used alongside complementary structural and textural characterisation methods, employing both ex situ and in situ approaches. In Chapter 2, we demonstrated that the complementary use of SANS and (U)SAXS is essential for SPC systems because each method provides different SLD contrasts. Under HME conditions, moderately unfolded yet globular proteins assemble into nano-aggregates that grow in the screw section until reaching a stable size (∼30 nm); these aggregates likely serve as building blocks for protein fibrils. Alignment of protein fibrils is observed at length scales larger than individual nano-aggregates, with greater alignment in the cooling die, especially near the colder walls. In Chapter 3, we confirmed that nm-to-µm scale alignment is already established within the extruder barrel, whereas the development of the lamellar phase-separated structure takes place predominantly in the cooling die. The use of a breaker plate promotes a more homogeneous distribution of structure and temperature in the protein melt. In Chapter 4, we investigated the effect of pH-shifting on multiscale structural anisotropy and texture. pH-shifting was shown to have a pronounced, non-monotonic effect. At the nm scale, SAS indicates that at pH > pI protein nano-aggregates increase in size and a transition from particulate to fibrillar-like aggregation takes place. Aligned lamellar structures emerge at pH > pI and diminish again at pH ≫ pI. In Chapter 5, we clarified mechanisms of structure formation by linking the heat-set gelation behaviour of fractionated 11S and 7S soy globulins to the multiscale structures. 7S globulin promotes deformability and facilitates flow-induced alignment, whereas 11S globulin contributes to network strengthening through covalent crosslinks. In Chapter 6, we developed a custom ‘neutron-transparent’ cooling die to enable direct in situ SANS measurements. The results show that protein aggregation in the extruder barrel is primarily governed by protein charge. Notably, partial relaxation of nanoscale alignment in the cooling die does not eliminate the macroscopic lamellar pattern, demonstrating that the final mm-scale structure reflects the early alignment state established at the cooling die entrance.